The Basilica of Saint-Denis (French: Basilique royale de Saint-Denis , now formally known as the Basilique-cathédrale de Saint-Denis [1] ) is a large former medieval abbey church and present cathedral in the commune of Saint-Denis , a northern suburb of Paris. The building is of singular importance historically and architecturally as its choir , completed in 1144, is widely considered the first structure to employ all of the elements of Gothic architecture . [2] [3]